This invention relates to aluminum base die casting alloys which provide good mechanical properties with conventional die casting machines without the need for heat treatment.
Aluminum alloys are now being widely used in manufacturing industries, and particularly the automotive industry, as a lightweight alternative to ferrous materials. In addition to having good strength characteristics, these aluminum alloys must have good die casting characteristics and be readily machinable.
A typical aluminum alloy for this purpose is an Alxe2x80x94Mgxe2x80x94Si type alloy as described in Evans et al., U.S. Pat. No. 5,573,606, issued Nov. 12, 1996. This aluminum alloy typically contains about 2.5-4.0 wt. % magnesium, 0.2-0.4 wt. % manganese, 0.25-0.6 wt. % iron and 0.2-0.45 wt. % silicon. This alloy is preferably cast in a vertical-type die casting machine.
Chamberlain et al. xe2x80x9cA Natural Aging Aluminum Alloy, Designed for Permanent Mold Usexe2x80x9d, AFS Transactions, Vol. 111, p. 133-142 (1977) describes the use of Alxe2x80x94Mgxe2x80x94Zn alloys for producing castings in a permanent mold. A typical alloy for this purpose contains about 3.3 wt. % magnesium, 2.9 wt. % zinc and 0.06 wt. % titanium. It was found that this alloy has limited use for some low-pressure die casting work.
An Alxe2x80x94Mgxe2x80x94Zn alloy particularly intended for die casting is described in Takeuchi et al., Japanese Patent Publication S61-28739, laid open Jul. 2, 1986. This alloy contains 0.5 2.5 wt. % zinc, 0.5-3.0 wt. % magnesium, 0.2-1.2 wt. % silicon, 0.2-1.5 wt. % iron, 0.1-1.2 wt. % manganese and the balance aluminum and incidental impurities.
Standard die castings exhibit far too low a ductility, e.g. about 2-3%, to be considered useful for structural applications. There are advanced, and expensive, die casting techniques and alloys that help the problem. However, they also require a heat treatment step which further adds to the cost. There is, therefore, a need for alloys and/or procedures which can provide good mechanical properties with conventional die casting machines without the need for heat treatment.
It is an object of the present invention to produce an aluminum alloy having low contents of copper, silicon and iron and which when cast in a conventional die casting machine has excellent as-cast strength without any need for heat treatment.
It is a further object of the present invention to produce an aluminum alloy having low contents of copper, silicon and iron and which when cast in a conventional die casting machine has improved as-cast ductility.
According to one embodiment of this invention, an aluminum alloy for die casting comprises 2.75-5.25 wt. % magnesium, 1.85-3.15 wt. % zinc, 0.65-1.2 wt. % manganese, 0.10-0.18 wt. % iron, less than 0.10 wt. % copper, less than 0.10 wt. % silicon, less than 0.20 wt. % titanium and the balance aluminum and incidental impurities where the ratio of weight percent Mg to weight percent Zn is greater than or equal to 1. This alloy exhibits improved strength and ductility after die casting and age hardening without a heat treatment when compared to similar die casting alloys based on higher Fe and lower Mn.
According to a further embodiment of this invention, an aluminum alloy for die casting comprises 4.75-5.25 wt. % magnesium, 2.85-3.15 wt. % zinc, 0.65-1.2 wt. % manganese, 0.10-0.18 wt. % iron, less than 0.10 wt. % copper, less than 0.10 wt. % silicon, less than 0.20 wt. % titanium and the balance aluminum and incidental impurities. This alloy exhibits excellent as-cast strength properties. After die casting and age hardening and without a heat treatment, the die cast product has a 0.2% offset yield strength (YS) of at least about 170 MPa, an ultimate tensile strength of at least about 280 MPa and an elongation value of at least about 5%.
According to yet a further embodiment of this invention, an aluminum alloy for die casting comprises 2.75-3.25 wt. % magnesium, 1.85-2.5 wt. % zinc, 0.65-1.2 wt. % manganese, 0.10-0.18 wt. % iron, less than 0.10 wt. % copper, less than 0.10 wt. % silicon, less than 0.20 wt. % titanium and the balance aluminum and incidental impurities. Compared to the above alloy, this one provides quite good strength properties together with excellent as-cast ductility. After die casting and age hardening and without a heat treatment, the die cast product has a 0.2% offset yield strength of at least about 130 MPa, an ultimate tensile strength of at least about 240 MPa and an elongation value of at least about 12%.
It is a surprising feature of this invention that with very low levels of copper, silicon and iron, the presence of manganese in place of iron serves to increase the yield strength and ultimate tensile strength without having an adverse effect on the ductility (elongation).
Considerable is known about the purpose of the various components in the above alloys and, for instance, magnesium is used to enhance the tensile strength of the alloy. Zinc also improves the alloy strength, while improving fluidity during casting. When magnesium and zinc are selected within the range of the present invention, a useful combination of strength and ductility is obtained. If the magnesium and/or zinc is higher than the inventive range, then the ductility is reduced to an unacceptable level and if the magnesium and/or zinc is less than the inventive range, then the strength is too low, even when enhanced by the manganese.
The amount of magnesium should be greater than or equal to the amount of zinc. It is believed that with magnesium and zinc levels of the present invention and with magnesium greater than or equal to zinc, an Alxe2x80x94Mgxe2x80x94Zn phase forms which creates fine precipitates even at room temperature, and these contribute to the increased strength but do not adversely affect the elongation. A level of magnesium greater than or equal to the level of zinc also increases the resistance to corrosion and hot shortness.
Within the broad range of magnesium and zinc it is possible to provide levels of magnesium or zinc that further enhance the strength or the ductility, but in both cases, the present of manganese at low iron, etc. provides further enhancement of the strength without being detrimental to the ductility.
Iron is typically added to counteract die soldering and manganese counteracts some negative effects of the iron as well as, itself, counteracting die soldering. In the present alloy, iron must be kept low (less than 0.18% by weight) since the combined effect at a higher iron with the manganese would be negative on the ductility.
Too much silicon can be up magnesium as Mg2Si that prevents formation of the desired Alxe2x80x94Mgxe2x80x94Zn phase. Titanium may be totally absent from the die casting alloys and is generally present in less than 0.12%. Low levels of copper are preferred to help avoid corrosion.
The alloys of this invention are useful for forming light weight die cast articles having as-cast mechanical properties superior to standard die cast alloys without the need for heat treatment. They are particularly useful for the production of structural and high integrity die castings for the automobile industry.